Deutsche Nationalbibliothek (German National Library) and hbz are working on a service1 which aims to facilitate the linking of data from different catalogues (whether library catalogues or union catalogues) by providing matching information about different records.

For this project we - amongst other things - need to link catalog entries to the bibliograhic identifiers in them. (There are a lot of identifiers in the realm of bibliographic data: widely known identifiers like ISBN, ISSN, DOI, URN, Handle, OCLC number, LCCN and many more local, regional and national identifiers. Some of them identify a bibliographic record while others identify the described bibliographic entity - which doesn't make it easier in a LOD context.)

The question here is how to represent a link in RDF from a record or a bibliographic resource to the associated identifiers. We have to decide between 1.) using individual predicates for each identifier or 2.) using the global predicate dc:identifier and characterizing the identifier more precisely through an xsd data type.

I just came upon the "Custom Datatype" part of Ian Davis' and Leigh Dodds' book "Linked Data Patterns" which gives a very good discussion of this question. It really helped me to understand this even better than after all your answers.

I would prefer to define specific sub properties of dc:identifier for that issue, e.g., as it is already done in the Bibliographic Ontology*, for example, bibo:isbn. I guess, the query time would be a bit shorter in this case.
Furthermore, I do not really understand, why you would separate the ISBN identifier. I think this is not really necessary.

*) Their super property for that issue is called bibo:indentifier (who knows why it is not aligned to dc:indentifier)

PS: You do not create an "XSD". However, you utilize an identifier for a datatype. This can be a predefined one from the XSD namespace or a selfdefined one from your own (/any other) namespace.

Thanks, I will contact both the bibo mailing list and W3C LLD-Group and call their attention to this question.
@zazi:"Furthermore, I do not really understand, why you would separate the ISBN identifier." --> I do this because in the LOD world you differ between a bibliographic resource and the bibliographic record that describes it. An ISBN identifies a bibliographic resource while identifiers in a catalog identify the record not the resource. Although there is no difference made in traditional cataloging (and Bibo doesn't make this difference too), I think it is reasonable...

Actually, if you use literal identifiers, it doesn't matter that much whether you enrich them with a property or a datatype. In both cases it requires additional domain knowledge to make use of them beyond a simple display for human consumption. You could infer some kind of identity if two resources have the same identifier of the same datatype and/or the same property, but I doubt that this is done for the general case in practice. But there is also:

3. The "URI approach"

There already are URI forms of many identifier systems and for the rest you can create URI schemes. Not all of them are HTTP URIs, but preferring literal strings over non-HTTP-URIs seems to contradict the whole idea of RDF.

However, from the example, one can see that the application mints its own URIs for the bibliographic resources anyway. So, I think, there is nothing wrong, to add an identifier, such as ISBN, as a literal. Creating same-as relations is another (orthogonal) task in my mind.

There is nothing wrong to add identifiers as literals, but there is little value, because these identifiers do not uniquely identify a resource within the scope of RDF, but only in prorietary contexts that are unrelated to RDF.

I think if the identifiers can already uniquely identify a resource outside of a Semantic Web context, then they will also do so in a knowledge representation that is powered by RDF, or? Please remember a unique name assumption cannot delivered by the Semantic Web (see, e.g., http://www.semanticoverflow.com/questions/1837/common-semantic-web-misconceptions-youve-encountered/1838#1838). URIs can be as ambigous as any other identifier type.

A basic assumption of RDF is that one URI reference always refers to one resource (no homonyms) in any context where RDF data is used. Without Unique Name Assumption there can be other URI references for the same resource (synonyms), but that's less critical. A literal identifier in contrast refers to one resource only in a specific context. Literal identifiers are just like names: in your family a given name may be unique, but among more people it is unlikely to identify one person.

Ed already mentioned to use URI where they already exist and we'll consider this. But I don't think that it makes sense to mint even more new URIs in the case none exist.
In fact, identifier+datatype is as much a unique name as a URI is. (And one might argue that every single literal is a unique identifier in the sense that its spelling is unique in contrast to every differently spelled literal. Uniqueness of identifiers as you postulate it is a question of their use and has nothing to do with the utter form of the identifier.)